Members PRIVATE by default, PROTECTED only if needed

Make attributes, methods, and other class members PRIVATE by default.

Make them only PROTECTED if you want to enable sub-classes that override them.

Internals of classes should be made available to others only on a need-to-know basis. This includes not only outside callers but also sub-classes. Making information over-available can cause subtle errors by unexpected redefinitions and hinder refactoring because outsiders freeze members in place that should still be liquid.